A duplex stainless steel contains a large amount of Cr, Mo, Ni and N, and intermetallic compounds and nitrides are easily precipitated.
Therefore, a solution heat treatment was carried out at a temperature of 1000° C. or higher so as to solid-solubilize the precipitates; and thereby, the duplex stainless steel was produced as a hot rolled steel material. In addition, when manufacturing a clad steel plate including the duplex stainless steel as a cladding material, the following techniques were applied.
A technique in which a chemical composition of a carbon steel is designed so as to maintain mechanical characteristics through a heat treatment at a high temperature of 1000° C. or higher, and this carbon steel is used as a base metal (Patent Document 1 and the like).
A technique in which hot rolling conditions are controlled; and thereby, a duplex stainless clad steel plate is produced without a heat treatment (Patent Document 2 and the like).
A technique in which reheating is carried out during hot rolling; and thereby, precipitation in a cladding material is suppressed (Patent Document 3 and the like).
By the way, in recent years, alloying element-saving duplex stainless steel has been developed in which contents of Ni, Mo and the like are saved, and a steel having a greatly decreased precipitation sensitivity of intermetallic compounds has been put into practical use.
Chromium nitrides are precipitates having a major influence on the material quality of the alloying element-saving duplex stainless steel. The chromium nitrides are precipitates generated by the combination of Cr and N, and, in the duplex stainless steel, cubic CrN or hexagonal Cr2N is frequently precipitated in ferrite grains or ferrite grain boundaries. In the case where these chromium nitrides are generated, impact property degrades, and corrosion resistance degrades due to a chromium-depleted layer generated by the precipitation.
The present inventors clarified the relationship between the precipitation of the chromium nitrides and component composition, and designed material qualities based on an idea of suppressing the precipitation of the chromium nitrides by controlling the component composition. As a result, the inventors invented and disclosed an alloying element-saving duplex stainless steel having favorable corrosion resistance and favorable impact property (Patent Document 4). Particularly, the content of Mn is increased; and thereby, the precipitation of the chromium nitrides is suppressed, and this technique is reflected in the component design of new alloying element-saving duplex stainless steel. And, due to its low cost and excellent characteristics such as corrosion resistance, the alloying element-saving duplex stainless steel has already been in use in a variety of fields.
The alloying element-saving duplex stainless steel is also expected to be used as a hot rolled steel plate. Generally, the hot rolled steel plate has a large thickness, since the hot rolled steel plate is not subjected to cold rolling, and the hot rolled steel plate is used particularly in a field where strength or toughness is required. Examples thereof include seawater desalination instruments, tanks in a shipping vessel and the like, and in the past, austenitic stainless steel was used in the majority of the above-described uses.
However, the duplex stainless steel generally has a higher strength than austenitic stainless steel; and therefore, the duplex stainless steel is advantageous in that the duplex stainless steel can be thinned while maintaining a necessary strength. In addition, since the used amount of expensive elements is small, the cost is low. For the above-described reasons, the duplex hot rolled stainless steel material has already been in use in some of the above-described uses.
On the other hand, toughness is a problem when the duplex hot rolled stainless steel material is used. That is, the duplex stainless steel includes a ferrite phase together with an austenite phase which is generally considered not to cause brittle fracture. Therefore, the duplex stainless steel exhibits the same ductile-brittle transition as that of a ferritic stainless steel in terms of impact toughness, and the duplex stainless steel has poor toughness compared to an austenitic stainless steel.
A number of studies have thus far been made regarding the above-described problem. The inventors clarifies the relationship between a steel structure and the toughness of the duplex hot rolled stainless steel plate in Patent Document 5, and the inventors discloses that the toughness can be improved by controlling a chemical composition and a heat treatment method.
In addition, the inventors newly paid attention to the application of the above-described alloying element-saving duplex stainless steel to a cladding material of a clad steel plate, and research and development were carried out. In the clad steel plate, corrosion resistance is given to the stainless steel used as the cladding material, and strength, toughness, and weldability are given to the base metal. Therefore, the clad steel plate is a hot rolled steel in which multiple characteristics can be obtained economically.
The clad steel plate is used at a portion at which the stainless steel and the base metal are structurally joined, and the clad steel plate generally has a large thickness, and is used particularly in a filed where strength or toughness is required. Examples thereof include seawater desalination instruments, tanks in a shipping vessel and the like, and, in the past, an austenitic stainless steel was used as a cladding material in the majority of the above-described uses.
However, there is an ongoing trend for the stainless steel in the above-described uses to be changed to a cheap duplex stainless steel, and there is an increasing demand for a cheaper clad steel plate including a duplex stainless steel as the cladding material.
By the way, in a production process of a duplex hot rolled stainless steel plate described in Patent Document 5 and in a production process of a clad steel plate of the related art, a solution heat treatment is indispensable. In order to solve the problem caused by the inter metallic compound or the chromium nitrides that degrade the corrosion resistance and the problem of toughness in the duplex stainless steel as described above, a solution heat treatment is required. Particularly, the alloying element-saving duplex stainless steel, which is the subject of the invention, has a property that easily allows the precipitation of nitrides in a temperature range of hot working, and the chromium nitrides are dispersed in the steel material when hot rolling is completed. Thereby, the impact property and the corrosion resistance degrade.
In the production process of the alloying element-saving duplex stainless steel, the chromium nitrides can be removed by carrying out the solution heat treatment. However, since the solution heat treatment decreases the strength, it can be also said that the solution heat treatment is not preferable in consideration of the uses of the hot rolled steel plate. Similarly, in the production process of the clad steel plate, the chromium nitrides in the cladding material can be removed by carrying out the solution heat treatment. However, in the case where the solution heat treatment is carried out at a temperature of 1000° C. or higher, the toughness of the base metal degrades; and therefore, the solution heat treatment is not preferable in consideration of the uses of the clad steel plate.
In addition, from the demand for the additional saving of costs or the recent demand for the saving of energy used, there is a demand for the saving of production costs or energy necessary for production through the removal of the solution heat treatment.